Filter Circuit and Semiconductor Device

ABSTRACT

A filter circuit includes a low-pass filter and a calibration circuit calibrating a frequency characteristic of the low-pass filter. The calibration circuit includes a negative feedback circuit and a control circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2007-145061, filed on May 31, 2007, theentire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

The present embodiment relates to a filter circuit and a semiconductordevice including a filter circuit which may include a calibrationfunction of a filter circuit.

2. Description of the Related Art

In a communication semiconductor device, a demodulated signal greatlydeviates from an ideal signal since characteristics of a filter in an IFcircuit which processes a received signal vary due to manufacturingprocesses, and this worsens a bit error rate and thus necessitates thecalibration of the characteristics of the filter.

As a conventional art to calibrate characteristics of a filter, therehas been known a method in which an oscillation circuit is formed byusing a replica circuit of the filter, characteristics of the replicacircuit are controlled so that the oscillation circuit has a constantoscillation frequency, and the characteristics of the filter arecontrolled similarly to the characteristics of the replica circuit(first conventional art: see, for example, Japanese Unexamined PatentApplication Publication No. 2002-100962, and P. Quinlan et al., “AMultimode 0.3-200-kb/s Transceiver for the 433/868/915-MHz Bands in0.25-μm CMOS”, IEEE J. Solid-State Circuits, vol. 39, no. 12, December2004). As another conventional art, there has been known a method inwhich characteristics of a filter are controlled so that a delay time ofa circuit including the filter becomes constant (second conventionalart: see, for example, Japanese Unexamined Patent ApplicationPublication No. 2006-287900). As still another conventional art, therehas been known a method in which a band-pass filter functions as aclosed-loop oscillation circuit and characteristics of the band-passfilter are controlled so that the oscillation circuit has a constantoscillation frequency (third conventional art: see, for example,Japanese Unexamined Patent Application Publication No. 2001-274654).

In the first conventional art, due to the need for providing the replicacircuit, chip size and power consumption of a semiconductor devicehaving the filter mounted thereon increases. Further, since thecharacteristics of the filter are controlled based on information whichis indirectly obtained by using the replica circuit, calibrationaccuracy is lower than when the characteristics of the filter arecontrolled based on information directly obtained from the filter.

In the second conventional art, delay time measurement is used, but whena digital circuit is operated by the same clock frequency, the use ofthe delay time measurement results in lowered calibration accuracy sinceit is more difficult to highly accurately execute the delay timemeasurement than to execute the frequency measurement. There is anotherproblem that this art is not directly applicable to a complex band-passfilter.

In the third conventional art, a center frequency and a Q-value of theband-pass filter, which determine the frequency characteristics of theband-pass filter, have to be separately calibrated since a gain and theQ-value of the band-pass filter need to have a proportional relation.Further, an amplitude detection circuit has to be provided to calibratethe Q-value of the band-pass filter, which increases circuit scale ofthe band-pass filter.

SUMMARY

It is an aspect of the embodiments discussed herein to provide a filtercircuit mounted on a semiconductor device includes a low-pass filter anda calibration circuit calibrating a frequency characteristic of thelow-pass filter. The calibration circuit may include a negative feedbackcircuit and a control circuit.

These together with other aspects and advantages which will besubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram illustrating a first embodiment.

FIG. 2 is an explanatory diagram illustrating details of low-passfilters in FIG. 1.

FIG. 3 is an explanatory chart illustrating the correlation between acount value of a counter and an oscillation frequency of a loop circuitin FIG. 2.

FIG. 4 is an explanatory chart illustrating a calibration sequence ofthe low-pass filter in FIG. 2.

FIG. 5 is an explanatory diagram illustrating a second embodiment.

FIG. 6 is an explanatory chart illustrating a frequency characteristicof a complex band-pass filter in FIG. 5.

FIG. 7 is an explanatory chart illustrating a calibration sequence ofthe complex band-pass filter in FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments will be described with reference to thedrawings.

FIG. 1 illustrates a first embodiment. A communication semiconductordevice 100 of the first embodiment includes a low noise amplifier (LNA)101 provided in a radio receiver circuit, mixers 102, 103, an oscillator104, a local oscillation signal generator 105, and low-pass filters 106,107. The mixer 102 mixes a high frequency signal supplied via the lownoise amplifier 101 and one of local oscillation signals supplied fromthe local oscillation signal generator 105 to output the resultant tothe low-pass filter 106. The mixer 103 mixes the high frequency signalsupplied via the low noise amplifier 101 and the other of the localoscillation signals supplied from the local oscillation signal generator105 to output the resultant to the low-pass filter 107. The localoscillation signal generator 105 generates the pair of local oscillationsignals with a 90° phase difference, based on an oscillation signalsupplied from the oscillator 104. The low-pass filter 106 filters outunnecessary frequency components in the output signal of the mixer 102to output the resultant. The low-pass filter 107 filters out unnecessaryfrequency components in the output signal of the mixer 103 to output theresultant. Further, the low-pass filters 106, 107 have a calibrationfunction (CAL) for calibrating frequency characteristics.

FIG. 2 illustrates details of the low-pass filters in FIG. 1. Each ofthe low-pass filters 106, 107 includes a switch circuit 151, a filter152, a comparator (CMP) 153, a buffer (BUF) 154, a counter 155, and afilter characteristics controllers 56. When a switch control signal SWCsupplied from the filter characteristics controller 156 is set low, theswitch circuit 151 selects input signals INP, INN to output them asfilter input signals INPS, INNS to the filter 152. On the other hand,when the switch control signal SWC is set high, the switch circuit 151selects feedback signals FBP, FBN supplied from the buffer 154 to outputthem as the filter input signals INPS, INNS to the filter 152.

The filter 152 filters out unnecessary frequency components in thefilter input signals INPS, INNS supplied from the switch circuit 151 tooutput the resultant as filter output signals OUPT, OUTN. Frequencycharacteristics of the filter 152 are variably set according to a filtercharacteristics control signal FLC supplied from the filtercharacteristics controller 156. Further, the filter 152 is configured sothat its Q-value is determined by an element-value ratio (ratio ofelement values) of internal elements (resistor elements and capacitorelements). The comparator 153 compares voltages of the filter outputsignals OUTP, OUTN supplied from the filter 152 and inverts a digitaloutput signal CMPOUT in response to a change of a magnitude relationbetween the filter output signals OUTP, OUTN. The buffer 154 generatesthe feedback signals FBP, FBN from the digital output signal CMPOUT ofthe comparator 153 to output them to the switch circuit 151.

When the switch control signal SWC is set high, the switch circuit 151,the filter 152, the comparator 153, and the buffer 154 form a loopcircuit. The comparator 153 and the buffer 154 are coupled so as to givea negative feedback to the filter 152, an output part of the buffer 154is configured so as to make a gain of the loop circuit larger than one,and the loop circuit oscillates with a frequency so that the filterinput signals INPS, INNS and the filter output signals OUTP, OUTN have a180° phase difference. The loop circuit oscillates independently of theQ-value of the filter 152. The oscillation frequency of the loop circuitis determined by the frequency characteristics of the filter 152, andtherefore, by adjusting the frequency characteristics of the filter 152so that the oscillation frequency of the loop circuit becomes constant(so that the oscillation frequency of the loop circuit falls within apredetermined range), it is possible to calibrate the frequencycharacteristics of the filter 152. For example, the filter 152 is afourth-order Butterworth low-pass filter, and the oscillation frequencyof the loop circuit is equal to a cut-off frequency of the filter 152.

In response to a rising transition (transition from low to high) of acounter control signal CST supplied from the filter characteristicscontroller 156, the counter 155 performs a count operation synchronizingwith a reference clock signal CKR during a predetermined period which isset based on a period of the digital output signal CMPOUT of thecomparator 153 (an oscillation period of the loop circuit). Uponcompletion of the count operation, the counter 155 outputs a count valueCOUNT to the filter characteristics controller 156. Further, the counter155 initializes the count value COUNT to zero in response to a fallingtransition (transition from high to low) of the counter control signalCST.

The filter characteristics controller 156 sets the switch control signalSWC high when a mode signal MD is set high, and sets the switch controlsignal SWC low when the mode signal MD is set low. The mode signal MD isset low when the low-pass filters 106, 107 are in a normal mode, and isset high when the low-pass filters 106, 107 are in a calibration mode.Further, when the mode signal MD is set high, the filter characteristicscontroller 156 controls the counter 155 by way of the counter controlsignal CST so that the counter 155 performs the count operation, therebyobtaining the count value COUNT, and adjusts the frequencycharacteristics of the filter 152 by way of a filter characteristicscontrol signal FLC so that the count value COUNT falls within a rangefrom a first reference value COUNT1 to a second reference value COUNT2(COUNT2>COUNT1). By setting an execution period of the count operationof the counter 155 in a plurality of periods of the digital outputsignal CMPOUT of the comparator 153, it is possible to improveresolution of the oscillation frequency of the loop circuit.

FIG. 3 illustrates the correlation between the count value of thecounter and the oscillation frequency of the loop circuit in FIG. 2. InFIG. 3, the count value COUNT of the counter 155 is taken on thevertical axis and the oscillation frequency f of the loop circuit istaken on the horizontal axis. As illustrated in FIG. 3, the higher theoscillation frequency f of the loop circuit becomes, the smaller thecount value COUNT of the counter 155 becomes. For example, in thelow-pass filters 106, 107, the frequency characteristics of the filter152 are adjusted so that the oscillation frequency f of the loop circuitfalls within a range from a lower limit value fl to an upper limit valuefh with a target value ft being a center therebetween. Therefore, thefirst reference value COUNT1 used by the filter characteristicscontroller 156 corresponds to the count value COUNT that the counter 155outputs when the oscillation frequency f of the loop circuit has theupper limit value fh. The second reference value COUNT2 used by thefilter characteristics controller 156 corresponds to the count valueCOUNT that the counter 155 outputs when the oscillation frequency f ofthe loop circuit has the lower limit value fl.

FIG. 4 illustrates a calibration sequence of the low-pass filter in FIG.2.

At step S101, the filter characteristics controller 156 changes theswitch control signal SWC from low to high in response to the transitionof the mode signal MD from low to high. Accordingly, the switch circuit151 selects the feedback signals FBP, FBN to output them as the filterinput signals INPS, INNS. As a result, the switch circuit 151, thefilter 152, the comparator 153, and the buffer 154 form the loopcircuit, and the loop circuit performs an oscillation operation.Thereafter, the calibration sequence shifts to step S102.

At step S102, the filter characteristics controller 156 initializes thenumber of loop times LOOP to 1. Thereafter, the calibration sequenceshifts to step S103.

At step S103, the filter characteristics controller 156 determineswhether or not the number of loop times LOOP is larger than apredetermined value N. When the number of loop times LOOP is equal to orsmaller than the predetermined value N, the calibration sequence shiftsto step S104. On the other hand, when the number of loop times LOOP islarger than the predetermined value N, the calibration sequence shiftsto step S111.

At step S104, the filter characteristics controller 156 changes thecounter control signal CST from low to high. Accordingly, the counter155 performs the count operation synchronizing with the reference clocksignal CKR during a predetermined period which is set based on theperiod of the digital output signal CMPOUT of the comparator 153. Then,the filter characteristics controller 156 obtains the count value COUNTupon completion of the count operation of the counter 155. Thereafter,the calibration sequence shifts to step S105.

At step S105, the filter characteristics controller 156 changes thecounter control signal CST from high to low. Accordingly, the counter155 initializes the counter value COUNT to zero. Thereafter, thecalibration sequence shifts to step S106.

At step S106, the filter characteristics controller 156 determineswhether or not the count value COUNT obtained at step S104 is largerthan the first reference value COUNT1. When the count value COUNT isequal to or smaller than the first reference value COUNT1, thecalibration sequence shifts to step S107. On the other hand, when thecount value COUNT is larger than the first reference value COUNT1, thecalibration sequence shifts to step S108.

At step S107, the filter characteristics controller 156 changes thefrequency characteristics of the filter 152 by way of the filtercharacteristics control signal FLC so that the oscillation frequency ofthe loop circuit decreases. Thereafter, the calibration sequence shiftsto step S110.

At step S108, the filter characteristics controller 156 determineswhether or not the count value COUNT obtained at step S104 is smallerthan the second reference value COUNT2. When the count value COUNT isequal to or larger than the second reference value COUNT2, thecalibration sequence shifts to step S109. On the other hand, when thecount value COUNT is smaller than the second reference value COUNT2, thecalibration sequence shifts to step S111.

At step S109, the filter characteristics controller 156 changes thefrequency characteristics of the filter 152 by way of the filtercharacteristics control signal FLC so that the oscillation frequency ofthe loop circuit increases. Thereafter, the calibration sequence shiftsto step S110.

At step S110, the filter characteristics controller 156 increments thenumber of loop times LOOP. Thereafter, the calibration sequence shiftsto step S103.

At step S11, the filter characteristics controller 156 changes theswitch control signal SWC from high to low in response to the change ofthe mode signal MD from high to low. Accordingly, the switch circuit 151selects the input signals INP, INN to output them as the filter inputsignals INPS, INNS. As a result, the filter 152 performs the filteroperation. Here, the calibration sequence is completed.

In the first embodiment as described above, since the frequencycharacteristics of the filter 152 are calibrated without using a replicacircuit, a circuit having the calibration function can be small incircuit scale and low in power consumption, which can reduce chip sizeand power consumption of the semiconductor device 100. Further, sincethe method using the frequency measurement is adopted, it is possible tocalibrate the frequency characteristics of the filter 152 moreaccurately than when a method using delay time measurement is adopted.In this manner, in the first embodiment, the frequency characteristicsof the low-pass filters 106, 107 (filter 152) can be calibrated easilyand accurately. Moreover, since the loop circuit oscillatesindependently of the Q-value of the filter 152, the Q-value of thefilter 152 can be set arbitrarily and a cut-off frequency of the filter152 and a gain can be independently set. Further, the Q-value of thefilter 152 is determined by the element-value ratio of the internalelements, which can eliminate a need for the calibration of the Q-valueof the filter 152.

FIG. 5 illustrates a second embodiment. A communication semiconductordevice 200 of the second embodiment includes a low noise amplifier (LNA)201 provided in a radio receiver circuit, mixers 202, 203, an oscillator204, a local oscillation signal generator 205, and a complex band-passfilter (COMPLEX-BPF) 206. The low noise amplifier 201, the mixers 202,203, the oscillator 204, and the local oscillation signal generator 205are the same circuits as the low noise amplifier 101, the mixers 102,103, the oscillator 104, and the local oscillation signal generator 105in the first embodiment (FIG. 1).

The complex band-pass filter 206 is a filter capable of image removal byutilizing a phase difference between an I-side and a Q-side, andincludes low pass filters 207, 208 and I-side/Q-side feedback circuits209, 210. The low-pass filters 207, 208 have the same internalconfiguration as that of the low-pass filters 106, 107 in the firstembodiment (FIG. 2). The I-side/Q-side feedback circuits 209, 210 areconfigured so that a center frequency of the complex band-pass filter206 is proportional to a cut-off frequency of the low-pass filters 207,208. Further, the I-side/Q-side feedback circuits 209, 210 invalidateI-side/Q-side feedback signals when a mode signal MD (not shown) is sethigh. That is, the complex band-pass filter 206 performs a complexband-pass filter operation when the mode signal MD is set low, andperforms a low-pass filter operation when the mode signal MD is sethigh. Frequency characteristics (a center frequency and a band width) ofthe complex band-pass filter 206 and frequency characteristics (acut-off frequency) of the low-pass filters 207, 208 are determined by anelement-value ratio of internal elements.

FIG. 6 illustrates the frequency characteristics of the complexband-pass filter in FIG. 5. In FIG. 6, a frequency ω is taken on thehorizontal axis and a transfer function |H(jω)| is taken on the verticalaxis. A frequency characteristics curve of the complex band-pass filter(COMPLEX-BPF) 206 is a frequency characteristics curve of the low-passfilters (LPF) 207, 208 shifted in the frequency axis direction, and isexpressed by a center frequency ωshift and a band width BW. The bandwidth BW of the complex band-pass filter 206 is twice as large as acut-off frequency ωc of the low-pass filters 207, 208.

A frequency ratio of the center frequency ωshift of the complexband-pass filter 206 and the cut-off frequency ωc of the low-passfilters 207, 208 is determined by an element-value ratio of the internalelements and thus is little influenced by manufacturing variation.Therefore, in the complex band-pass filter 206 in which the ratio of thecenter frequency ωshift and the band width BW is determined by theelement-value ratio of the internal elements, in accordance with thecalibration of the cut-off frequency ωc of the low-pass filters 207,208, the center frequency ωshift and the band width BW of the complexband-pass filter 206 are also automatically calibrated.

FIG. 7 illustrates a calibration sequence of the complex band-passfilter in FIG. 5.

At step S201, the I-side/Q-side feedback circuits 209, 210 invalidatethe I-side/Q-side feedback signals in response to the transition of themode signal MD from low to high. Accordingly, the complex band-passfilter 206 performs the low-pass filter operation (LPF operation).Thereafter, the calibration sequence shifts to step S202.

At step S202, the calibration sequence of the low-pass filters 207, 208is performed similarly to the calibration sequence of the low-passfilters 106, 107 in the first embodiment (FIG. 4). Thereafter, thecalibration sequence shifts to step S203.

At step S203, the I-side/Q-side feedback circuits 209, 210 validate theI-side/Q-side feedback signals in response to the transition of the modesignal MD from high to low. Accordingly, the complex band-pass filter206 performs the complex band-pass filter operation (COMPLEX-BPFoperation). Here, the calibration sequence is completed.

In the second embodiment as described above, as in the calibrationsequence of the low-pass filters 106, 107 in the first embodiment, inaccordance with the calibration of the frequency characteristics (thecut-off frequency) of the low-pass filters 207, 208, the frequencycharacteristics (the center frequency and the band width) of the complexband-pass filter 206 are also automatically calibrated. Therefore, it ispossible to calibrate the frequency characteristics of the complexband-pass filter 206 easily and accurately.

The first and second embodiments describe the examples where the presentembodiment is applied to the calibration of the frequencycharacteristics of the IF filter in the communication semiconductordevice, but it should be noted that the present embodiment may beapplied to the calibration of frequency characteristics of a low-passfilter in a semiconductor device other than the communicationsemiconductor device.

It is a proposition of the aforementioned embodiment to provide an artfor realizing simple and accurate calibration of frequencycharacteristics of a filter.

According to an aspect of the embodiment, a filter circuit mounted on asemiconductor device includes a low-pass filter and a calibrationcircuit calibrating a frequency characteristic of the low-pass filter.The calibration circuit includes a negative feedback circuit and acontrol circuit. When the filter circuit is in a calibration mode, thenegative feedback circuit gives a negative feedback to the low-passfilter to form a loop circuit and sets a gain of the loop circuit largerthan one to make the loop circuit oscillate. When the filter circuit isin the calibration mode, the control circuit controls the frequencycharacteristic of the low-pass filter so as to make an oscillationfrequency of the loop circuit fall within a predetermined range.

For example, the negative feedback circuit includes a comparator, abuffer, and a switch circuit. The comparator generates a digital outputsignal from an output signal of the low-pass filter. The buffergenerates a feedback signal from the digital output signal of thecomparator. The switch circuit selects an input signal to output theinput signal as an input signal of the low-pass filter when the filtercircuit is in a normal mode, and selects the feedback signal to outputthe feedback signal as the input signal of the low-pass filter when thefilter circuit is in the calibration mode. The control circuit includesa counter and an adjustment circuit. When the filter circuit is in thecalibration mode, the counter performs a count operation during apredetermined period which is set based on a period of the digitaloutput signal of the comparator. When the filter circuit is in thecalibration mode, the adjustment circuit compares a count value of thecounter and a reference value to adjust the frequency characteristic ofthe low-pass filter according to a result of the comparison. Further,the loop circuit oscillates independently of a Q-value of the low-passfilter. A Q-value of the low-pass filter is determined by anelement-value ratio of internal elements. For example, the low-passfilter is a fourth-order Butterworth low-pass filter and an oscillationfrequency of the loop circuit is equal to a cut-off frequency of thelow-pass filter.

In the above-described aspect of the embodiment, since the frequencycharacteristic of the low-pass filter is calibrated without using areplica circuit, the calibration circuit can be small in scale and lowin power consumption, which can reduce chip size and power consumptionof the semiconductor device. Further, adopting the method using thefrequency measurement enables more accurate calibration of the frequencycharacteristic of the low-pass filter than adopting a method using delaytime measurement. In this manner, according to the aspect of theembodiment, it is possible to calibrate the frequency characteristic ofthe low-pass filter easily and accurately. Further, since the loopcircuit oscillates independently of the Q-value of the low-pass filter,the Q-value of the low-pass filter can be arbitrarily set, which makesit possible to set the cut-off frequency and a gain of the low-passfilter separately. Further, the Q-value of the low-pass filter dependson the element-value ratio of the internal elements, which can eliminatea need for the calibration of the Q-value of the low-pass filter.

The many features and advantages of the embodiments are apparent fromthe detailed specification and, thus, it is intended by the appendedclaims to cover all such features and advantages of the embodiments thatfall within the true spirit and scope thereof. Further, since numerousmodifications and changes will readily occur to those skilled in theart, it is not desired to limit the inventive embodiments to the exactconstruction and operation illustrated and described, and accordinglyall suitable modifications and equivalents may be resorted to, fallingwithin the scope thereof.

1. A filter circuit comprising: a low-pass filter; and a calibrationcircuit calibrating a frequency characteristic of the low-pass filter,wherein the calibration circuit includes: a negative feedback circuitwhich gives a negative feedback to the low-pass filter to form a loopcircuit and sets a gain of the loop circuit larger than one to make theloop circuit oscillate, when the filter circuit is in the calibrationmode; and a control circuit which controls the frequency characteristicof the low-pass filter so as to make an oscillation frequency of theloop circuit falls within a predetermined range, when the filter circuitis in the calibration mode.
 2. The filter circuit according to claim 1,wherein: the negative feedback circuit comprises: a comparatorgenerating a digital output signal from an output signal of the low-passfilter; a buffer generating a feedback signal from the digital outputsignal of the comparator; and a switch circuit which selects an inputsignal to output the input signal as an input signal of the low-passfilter when the filter circuit is in a normal mode, and selects thefeedback signal to output the feedback signal as the input signal of thelow-pass filter when the filter circuit is in the calibration mode; andwherein the control circuit comprises: a counter which performs a countoperation during a predetermined period which is set based on a periodof the digital output signal of the comparator, when the filter circuitis in the calibration mode; and an adjustment circuit which compares acount value of the counter and a reference value to adjust the frequencycharacteristic of the low-pass filter according to a result of thecomparison, when the filter circuit is in the calibration mode.
 3. Thefilter circuit according to claim 1, wherein the loop circuit oscillatesindependently of a Q-value of the low-pass filter.
 4. The filter circuitaccording to claim 1, wherein a Q-value of the low-pass filter isdetermined by an element-value ratio of internal elements.
 5. The filtercircuit according to claim 1, wherein: the low-pass filter is afourth-order Butterworth low-pass filter; and an oscillation frequencyof the loop circuit is equal to a cut-off frequency of the low-passfilter.
 6. The filter circuit according to claim 1, wherein: thelow-pass filter is a low-pass filter included in a complex band-passfilter; and a frequency characteristic of the complex band-pass filterand the frequency characteristic of the low-pass filter are determinedby an element-value ratio of internal elements; and calibration of thefrequency characteristic of the complex band-pass filter is realized bythe calibration of the frequency characteristic of the low-pass filter.7. A semiconductor device comprising a filter circuit, the filtercircuit comprising: a low-pass filter; and a calibration circuitcalibrating a frequency characteristic of the low-pass filter, whereinthe calibration circuit comprises: a negative feedback circuit whichgives a negative feedback to the low-pass filter to form a loop circuitand sets a gain of the loop circuit larger than one to make the loopcircuit oscillate, when the filter circuit is in a calibration mode; anda control circuit which controls the frequency characteristic of thelow-pass filter so as to make an oscillation frequency of the loopcircuit falls within a predetermined range, when the filter circuit isin the calibration mode.
 8. The semiconductor device according to claim7, wherein: the negative feedback circuit comprises: a comparatorgenerating a digital output signal from an output signal of the low-passfilter; a buffer generating a feedback signal from the digital outputsignal of the comparator; and a switch circuit which selects an inputsignal to output the input signal as an input signal of the low-passfilter when the filter circuit is in a normal mode, and selects thefeedback signal to output the feedback signal as the input signal of thelow-pass filter when the filter circuit is in the calibration mode; andwherein the control circuit comprises: a counter which performs a countoperation during a predetermined period which is set based on a periodof the digital output signal of the comparator, when the filter circuitis in the calibration mode; and an adjustment circuit which compares acount value of the counter and a reference value to adjust the frequencycharacteristic of the low-pass filter according to a result of thecomparison, when the filter circuit is in the calibration mode.